Transcript ch09c2wcr
Muscles and Muscle Tissue: Smooth Muscle Part C2 Prepared by Janice Meeking, W. Rose, and Jarvis Smith. Figures from Marieb & Hoehn 8th ed. Portions copyright Pearson Education Smooth Muscle Found in walls of most hollow organs (but heart is cardiac muscle) Often in two layers (longitudinal and circular) Microanatomy Spindle-shaped cells: thin and short compared with skeletal muscle fibers SR: less developed than in skeletal muscle Pouchlike infoldings (caveolae) of surface membrane sequester Ca2+ (instead of SR) No T tubules No sarcomeres or myofibrils Yes actin & myosin filaments No tendons; endomysium connects to surrounding tissue Smooth Muscle Innervation No tight neuromuscular junction (unlike skeletal muscle with its sophisticated NMJ) Autonomic nerve fibers innervate smooth muscle Varicosities (bulbous swellings) of nerve fibers release neurotransmitters broadly (diffuse junctions) Varicosities Autonomic nerve fibers innervate most smooth muscle fibers. Smooth muscle cell Synaptic vesicles Mitochondrion Varicosities release their neurotransmitters into a wide synaptic cleft (a diffuse junction).Figure 9.27 Myofilaments in Smooth Muscle • Thin and thick filaments; have heads along their entire length • No troponin complex; protein calmodulin binds Ca2+ • Myofilaments are spirally arranged, causing smooth muscle to contract in a corkscrew manner • Noncontractile intermediate filaments, anchored to membrane by dense bodies, help preserve cell shape during contraction Copyright © 2010 Pearson Education, Inc. Intermediate filament Caveolae Gap junctions Dense bodies Nucleus (a) Relaxed smooth muscle fiber Nucleus Dense bodies (b) Contracted smooth muscle fiber Copyright © 2010 Pearson Education, Inc. Copyright © 2010 Pearson Education, Inc. Figure 9.28 Contraction of Smooth Muscle • Actin & myosin sliding filament mechanism • Slow, synchronized contractions • Cells electrically coupled by gap junctions (in some tissues) • Rate, intensity of contraction regulated by neural and chemical stimuli • Final trigger is intracellular Ca2+ which comes from (sparse) SR and extracellular space (caveolae) Copyright © 2010 Pearson Education, Inc. Role of Calcium Ions in Smooth Muscle • Calcium binds to and activates calmodulin (a protein) • activates myosin light chain kinase enzymes (another protein) • Phosphorylates and activates myosin • activated heads form cross bridges with actin Much slower than E-C coupling in skeletal muscle Copyright © 2010 Pearson Education, Inc. Extracellular fluid (ECF) Ca2+ E-C coupling in smooth muscle Next slides show the details Plasma membrane Cytoplasm 1 Calcium ions (Ca2+) enter the cytosol from the ECF via voltagedependent or voltageindependent Ca2+ channels, or from the scant SR. Ca2+ 2 Ca2+ binds to and activates calmodulin. Sarcoplasmic reticulum Ca2+ Inactive calmodulin Activated calmodulin 3 Activated calmodulin activates the myosin light chain kinase enzymes. Inactive kinase 4 The activated kinase enzymes catalyze transfer of phosphate to myosin, activating the myosin ATPases. Activated kinase ATP ADP Pi Pi Inactive myosin molecule Activated (phosphorylated) myosin molecule 5 Activated myosin forms cross bridges with actin of the thin filaments and shortening begins. Thin filament Thick filament Figure 9.29 Extracellular fluid (ECF) Ca2+ Plasma membrane Cytoplasm 1 Calcium ions (Ca2+) enter the cytosol from the ECF via voltagedependent or voltageindependent Ca2+ channels, or from the scant SR. Ca2+ Sarcoplasmic reticulum Copyright © 2010 Pearson Education, Inc. Figure 9.29, step 1 2 Ca2+ binds to and activates calmodulin. Ca2+ Inactive calmodulin Copyright © 2010 Pearson Education, Inc. Activated calmodulin Figure 9.29, step 2 3 Activated calmodulin activates the myosin light chain kinase enzyme (MLCK). Inactive kinase Copyright © 2010 Pearson Education, Inc. Activated kinase Figure 9.29, step 3 4 The activated kinase enzymes catalyze transfer of phosphate to myosin (phosphorylation), activating the myosin. ATP ADP Pi Pi Inactive myosin molecule Copyright © 2010 Pearson Education, Inc. Activated (phosphorylated) myosin molecule Figure 9.29, step 4 5 Activated myosin forms cross bridges with actin of the thin Filaments, and shortening begins. Thin filament Thick filament Copyright © 2010 Pearson Education, Inc. Figure 9.29, step 5 Smooth muscle relaxation • Active export of Ca2+ from cytoplasm into SR and extracellular fluid (caveolae), which causes • Ca2+ detachment from calmodulin which leads to • Inactivation of MLCK, which allows • Dephosphorylation of myosin to “deactivate” it • Much slower than in skeletal muscle Regulation of Smooth Muscle Contraction • Neural regulation • • Neurotransmitter binding [Ca2+] in sarcoplasm; either graded (local) potential or action potential Response depends on neurotransmitter released and type of receptor molecules • Hormones and local chemicals regulate contraction • • Histamine, excess C02, pH, etc May either enhance or inhibit Ca2+ entry Special Features of Smooth Muscle Contraction Stress-relaxation response • Stretch causes brief contraction, then muscle adapts to new length • Retains ability to contract on demand • Stress-relaxation response enables organs such as stomach, bladder to expand significantly Length-tension relationship • Can generate contractile force when between half and twice its resting length (much wider range than skeletal muscle – why?) Special Features of Smooth Muscle Contraction Single unit • Cells connected by gap junctions so they all contract together • Prominent stress relaxation response • Walls of most hollow organs: gut, bladder, uterus… Multi-unit • Cells not connected by gap junctions • Allows finer control of force • Walls of large arteries, large airways, iris, arrector pili